In order to save energy, a known lighting device is configured to turn off a light source when a sensor device senses the presence of a person and to turned off (or dim) the light source when the sensor device does not sense the presence of a person. Document 1 (JP 2014-219249 A) describes an example of such a sensor device. The sensor device described in Document 1 includes a radio wave sensor formed as a Doppler sensor and a signal processor configured to perform signal processing on a sensor signal output from the radio wave sensor. The radio wave sensor (Doppler sensor) outputs a sensor signal having a frequency equal to a frequency difference between a transmission wave and a reflection wave to the signal processor. The frequency of the sensor signal has a value equal to a frequency proportional to a movement speed of an object which reflects off the radio wave (e.g., the speed at which a person walks) and an vibration frequency of an object staying at one position and vibrating. The signal processor converts (performs orthogonal transformation of) the sensor signal, which is a signal in a time domain, into a signal of a frequency domain. The signal processor presumes a noise component (background signal) which steadily appears among noise components included in the sensor signal. Then, the signal processor removes the background signal, which is presumed, from the sensor signal, thereby improving the sensing accuracy of a sensing target object (e.g., a moving person).
The sensor device is required to improve the sensing accuracy with a relatively simple process. However, in the sensor device described in Document 1, it has been difficult to simplify the signal processing of a signal processor while improving the sensing accuracy.